Each document, reference, patent application, or patent cited in this text is expressly incorporated herein in its entirety by reference, which means that it should be read and considered by the reader as part of this text. That the document, reference, patent application, or patent cited in this text is not repeated in this text is merely for reasons of conciseness.
The following discussion of the background to the invention is intended to facilitate an understanding of the present invention only. It should be appreciated that the discussion is not an acknowledgement or admission that any of the material referred to was published, known or part of the common general knowledge of the person skilled in the art in any jurisdiction as at the priority date of the invention.
In a multi-cell wireless network, there are multiple network access points, which may be fixed stations, with which devices, such as mobile devices or mobile stations, may establish a connection, in order to access the wireless network. For example, the wireless communication technology used may be Wi-Fi. The mobile device may establish a connection with a particular access point to access the wireless network. However, as the mobile device moves relative to the serving access point, the connection with the serving access point may be lost or dropped due to, for example, insufficient signal strength to support the connection. Therefore, in such a wireless network, there exists a handover procedure for triggering the mobile device to establish a new connection with a target access point providing better signal strength when the connection with the serving access point becomes unsatisfactory or less desirable in an attempt to maintain continuous communication with the wireless network.
Typically, a handover decision is made primarily based on the Received Signal Strength Indication (RSSI) of a radio signal, as seen by the mobile device or the access point, to determine when a handover should be triggered and the target access point the mobile device should be handed over to. For example, in G, P. Pollini, “Trends in Handover Design”, IEEE Communications Magazine, Vol. 34, pp. 82-90, March 1996, there were described various algorithms for deriving handover trigger conditions for a mobile device to change access points based on the RSSI determined by the mobile device and/or the network. For instance, a handover is triggered when the RSSI of a target access point is greater than the combined sum of the RSSI of the serving access point and a fixed handover margin.
In other related methods, a handover decision is alternatively made primarily based on the channel packet loss (Packet Loss) (i.e., the percentage of packets that are lost over a radio channel) or bit error rates (BER). In general, these methods characterise the quality of the communications channel in a scalar fashion and are reactive or lagging indicators.
In the above-described methods, the handover decision is ultimately made using a single scalar value (e.g., RSSI). However, the single scalar value alone is not always a reliable indicator to use in a radio environment when making handover decisions since it is subject to influences such as “radio fades” and multi-path reflections that together can provide false readings. For example, multipath fading characteristics are described in T. A. Wysocki and H. J. Zepernick, “Characterization of the indoor radio propagation channel at 2.4 GHz”, Journal of Telecommunications and Information Technology, No. 3-4, vol. 1, pp. 8490, 2000.
For instance, temporary poor signal strength may be detected at locations where there would normally be acceptable signal strength. These temporal variations are known as dips or “fades” in radio communications, which are usually transient in nature. As a result, when only a single scalar valve such as RSSI is relied upon, a handover decision may be unnecessarily triggered based on such temporary poor signal strength for example. For example, the inherent variability arising from using RSSI to make handover decision is discussed in R. Narasimhan and D. C. Cox, “A Handoff Algorithm for Wireless Systems Using Pattern Recognition”, in Proc. IEEE International Symp. on Personal, Indoor and Mobile Radio Commun. (PIMRC'98), pp. 335-339, September 1998.
RSSI methods tend to be reactive and conservative rather than predictive. This lack of predictive capability occurs at least in part because it is difficult to determine from the current RSSI what the RSSI will be a few seconds into the future for a mobile device that is in motion. For example, for RSSI based handover, RSSI from a candidate access point must be greater than current RSSI plus a safety margin. Consequently, handover decisions lag behind a moving user, resulting in a dropped or lost call if the user moves the mobile device away from the current access point too quickly.
Therefore, in the above-described methods, some handover decisions are suboptimal which may result in undesirable lost or dropped calls.
U.S. Pat. No. 7,536,186 discloses the use of speed and/or displacement information derived from an accelerometer or a Global Positioning System (GPS) chipset of a mobile device only for the purpose of determining whether to switch the mobile device's call connection from a wireless local area network (WLAN) coverage to a wide area network (WAN) coverage. In particular, if the speed and/or displacement information exceeds a predetermined threshold, then the mobile device's call connection with the WLAN coverage is switched to the WAN coverage. This is to increase the battery life of the device by making the mobile device run the hardware and software for only one of the WAN and WLAN systems at a time, instead of both simultaneously.
However, in U.S. Pat. No. 7,536,186, it is understood that the handover decision within a multi-cell network (such as within the WLAN network) is still made primarily based on RSSI. Therefore, U.S. Pat. No. 7,536,186 also suffers similar problems as discussed above when a handover decision is made primarily based on a single scalar value such as RSSI.
It is against this background that the present invention has been developed.